The present invention relates to a telescopic boom for lifting a load, such as is used in cranes and aerial platform apparatus.
Telescopic booms have long been used for lifting and moving loads, such as in connection with cranes, and have also been used where the load is, specifically, a workman supported in a workmans's basket or platform. Various constructions have heretofore been proposed and used, the primary desideratum in connection with such constructions being that they sustain the anticipated load, with any necessary factor of safety selected, and that the structure be as light as possible, consistent with safety.
One construction which has been used is the formation of tubular sections of the telescopic boom is two channels placed in facing relationship so as to form a box-like or tubular construction, with the free, longitudinally extending edges of the two channels being welded together, and with the welds being the top and bottom plates of the tubular boom section thereby formed. It is the conventional practice, after formation of the tubular boom sections, to provide them with suitable wear pads of low friction material, to permit the sections to slide telescopically one within another. These wear pads are known to be the part of the boom structure which transfers loads and forces from one boom section to another. In the placement of wear pads on the bottom plate of one boom section, to cooperate with the bottom plate of the adjacent telescopic boom section, it was found necessary with the above-described boom construction to place the wear pads inwardly from the side walls of the boom section. This was because of the radius required for the bending of the metal of which the channel was formed so as to provide webs and flanges extending generally perpendicularly. It is known that the radius of bend of such channels must be several times the thickness of the material. This inward location of the wear pads is known to cause bending of the bottom plate of the supported boom section, and to counteract this bending, the bottom plate was provided with an additional stiffening plate. Alternatively, because one-half of the bottom plate and one-half of the top plate, and the side wall were of a channel of uniform thickness, increasing the thickness of the bottom plate necessarily increased the thickness of the sides and top, thereby adding unnecessarily to the thickness and therefore to the weight of the boom section.
A number of the above discussed problems have been recognized in a proposal of a telescoping boom construction in which the tubular boom sections were made of a pair of vertical side walls or webs having their top and bottom edges welded to top and bottom plates or webs, the vertical webs being spaced inwardly from the longitudinal edges of the top and bottom webs. This construction of the box section completely eliminated radii present in the prior art structures as above described, the corners formed where the webs are joined by welding being square. The wear pads to support such a boom section, from the next larger boom section, were placed so as to be substantially directly over and under the side webs, the wear pads extending outwardly beyond the side walls or webs, engaging the upper surface of the top plate and the lower surface of the bottom plate, which top and bottom plates also extended substantially beyond the side walls or webs. This construction provides for the width of the boom sections being greater than the width between the outer surfaces of the vertical side walls or webs, thereby adding to the width and consequently the weight of the boom. This construction, also, provided uniform thickness of the four plates which were welded together to make up a boom section, and this construction further required a minimum of four longitudinally extending welds, each of which was provided to connect one web or plate to the web or plate adjacent and perpendicular to it.
There have also been disclosed, in the prior art, a boom having telescopic sections each comprising a bottom plate having a channel joined to it, as well as a boom made up of two essentially channel-shaped elements, the lower element being of greater thickness than the upper element.